The long-term objectives of this project are to determine the mechanisms involved in early T cell triggering by Toxoplasma gondii, and to determine the consequences of early T cell responses on subsequent pathogenesis of infection. Although the parasite under normal conditions induces a highly efficient immune response, amongst immunocompromised patients and congenitally infected infants T. gondii is a major opportunistic infection which can cause life-threatening disease. The studies in this proposal employ mice as an experimental animal model. Toxoplasmosis in mice follows a similar course to that occurring in humans. Earlier work on mouse T cells has established that the parasite possesses the superantigen-like ability to trigger nonimmune T lymphocytes and induce preferential expansion of Vbeta5+ T cells, in particular those of the CD8+ subset. Associated with this expansion is release of high levels of IFN- gamma, a cytokine which is essential for protection, but which when produced in large quantities may contribute to detrimental host pathology. This project will address the question of whether early T cell triggering for IFN-gamma production acts to the benefit or detriment of the host. There are four specific aims of this proposal. 1, To determine the IL-12 dependency of the Vbeta5+CD8+ response and the involvement of the latter cells in the nonpersistence of CD4+ T lymphocytes in culture, a phenomenon that may be related to the increased CD8/CD4 ratio often observed during human toxoplasmosis. 2, To evaluate the parasite's ability to induce a Vbeta5+ T cell-mediated cytokine shock response, and to determine if in vitro expanded cells induce a similar response, or whether they possess a protective capability. 3, To determine if virulent and avirulent parasite strains possess the ability to preferentially expand specific Vbeta families and/or CD8+ cells during primary culture. This may be related to the variable pathology associated with different parasite strains in mice and humans. 4, To biochemically characterize and purify the parasite molecule responsible for preferential triggering of Vbeta5+CD8+ T lymphocytes. These specific aims will be accomplished by employing techniques of cell separation and depletion, flow cytometric analyses, polymerase chain reaction amplification of cytokine mRNA, and protein immunochemistry.